A high speed turbo machine, such as, for example, a gas turbine, generally include a number of buckets arranged in axially oriented rows, the rows of buckets being rotated in response to the force of a high pressure fluid flowing axially through the machine. Due to their complex design, natural resonant mechanical frequencies of the buckets may coincide with or be excited by certain bucket rotational speeds and rotational harmonics thereof. Each natural frequency is associated with a particular mode shape, each of which is a different combination of vibrational deflections such as along the rotational axis of the turbine, perpendicular to the rotational axis of the turbine, and so forth.
In known systems for monitoring and analyzing vibrations in shrouded bucket structures, where the shroud is made integral with the bucket, the shroud may be provided with triggers that are placed in the shroud. One trigger is generally provided to each shroud portion associated with a bucket such that each trigger corresponds to a bucket. In most field test and on-line applications, a sensor may be provided for sensing the arrival of each trigger as the trigger passes the sensor. The data is analyzed on the basis of data identified with each trigger, i.e., each bucket, such that the vibration characteristics of each target location are individually analyzed, based on multiple rotations of the bucket row, to characterize the vibration characteristics of the coupled shroud structure. In such systems, however, it is difficult to discern the mode shape of the buckets.